(1) Field of the Invention
This invention relates to a receiving unit and semiconductor device and, more particularly, to a receiving unit for receiving CDMA system signals having a plurality of multipath components and semiconductor device for processing CDMA system signals having a plurality of multipath components.
(2) Description of the Related Art
The code division multiple access (CDMA) system is a strong candidate for next generation mobile communication systems and cellular telephones in which this system is adopted are gradually spreading into the market.
With the CDMA system, sending information for users by spreading with spreading codes which differ among different users enables a plurality of users to communicate in the same frequency band at the same time.
By the way, there is the problem of multipath fading in mobile communication. If multipaths are generated, desired signals on each path will reach at different times and their strength will correspond to their arrival times. A desired signal which arrives at each time is a multipath component.
The Rake receiving system will improve the characteristics of the spread spectrum communication system in this multipath environment. With the Rake receiving system, multipath components are demodulated and combined if multipath fading has occurred.
FIG. 10 is a block diagram showing the structure of a conventional receiving unit including a Rake combining section.
As shown in FIG. 10, a conventional receiving unit including a Rake combining section comprises an antenna 1, a receiving section 2, a control section 3, fingers 4-0 through 4-N, and a RAKE combining section 5.
The antenna 1 acquires electronic waves sent from a base station and supplies them to the receiving section 2.
The receiving section 2 converts electronic waves acquired by the antenna 1 into electrical signals and supplies them to the fingers 4-0 through 4-N.
The control section 3 calculates delay time for multipath signals and supplies signals indicative of the delay time to the fingers 4-0 through 4-N respectively.
Each of the fingers 4-0 through 4-N refers to a signal indicative of delay time, adjusts the phase of a spreading code for despreading a received signal, and demodulates a multipath component by despreading with this spreading code to obtain a desired signal.
The RAKE combining section 5 performs a time adjustment on desired signals output from the fingers 4-0 through 4-N and performs the maximal ratio combining of the signals to generate a demodulated signal.
Now, operation in the above conventional receiving unit will be described.
The antenna 1 acquires electronic waves which arrive from a base station via a plurality of paths (multipaths) and supplies them to the receiving section 2.
The receiving section 2 converts the received signal, being a radio frequency (RF) signal, into an intermediate frequency (IF) signal, generates, for example, 8-bit I and Q signals, and outputs them.
The electronic waves sent from the base station will arrive via the multipaths. Accordingly, as shown in FIG. 11, the received signal has a plurality of peaks. The control section 3 calculates delay time for multipath signals and supplies signals indicative of the delay time to the fingers 4-0 through 4-N respectively.
Each of the fingers 4-0 through 4-N refers to the signal indicative of delay time supplied from the control section 3, delays a despreading code by predetermined time, and performs despreading by multiplying the signal supplied from the receiving section 2 and the despreading code together.
The Rake combining section 5 performs a time adjustment on signals output from the fingers 4-0 through 4-N after the despreading and performs the maximal ratio combining of the signals to generate a demodulated signal.
As stated above, by using Rake combining, a plurality of peaks can be united into one, that is to say, scattered power can be combined. As a result, an S/N ratio will be improved.
As shown in FIG. 10, however, with conventional Rake receiving units, the number of fingers which need to be located correspond to that of multipaths, resulting in large-scale circuits.
Moreover, as shown in FIG. 12, if the timing of a received signal changes and is advanced significantly in a predetermined finger, that portion of the signal will not be recovered correctly. In the worst case, a symbol will be lost.
For example, as a result of the movement of a receiving unit (or the movement of a reflection object which reflects electronic waves), a path on which a finger is performing despreading disappears and the same finger must perform despreading on another path component. In such a case, as shown in the third and fourth slots in FIG. 12, if a path component to be newly processed has already been received, that portion of a signal will be lost.